The present invention relates to a method of effectively trapping ions produced external to an ion trapping device, namely the quadrupole ion trap.
The quadrupole ion trap was initially described by Paul et al. in U.S. Pat. No. 2,939,952 and normally consists of three electrodes; a ring electrode and two end-cap electrodes one on each side of the ring electrode. The electrodes all have rotationally-symmetric hyperbolic surfaces and are aligned on the same axis. The electrodes enclose a trapping region and a radio-frequency (RF) voltage is normally applied to the ring electrode to establish a trapping field. A variety of quadrupole ion traps, having stretched geometries or having hyperbolic surfaces with inclined asymptotes, are used in commercial mass spectrometers which utilize the quadrupole ion trap as an ion trapping device. Recent use of external ion sources coupled to the quadrupole ion trap have enabled access to a wide range of applications, such as liquid chromatography and matrix-assisted laser desorption/ionization (MALDI). The ions produced by these external ion sources have a range of initial ion energies at the sample surface, or in the sample ionization region. Problems arise due to the fact that a quadrupole ion trap operating at a high RF voltage will only accept ions which arrive at the entrance hole in one of the electrodes within a narrow phase range of the RF voltage. Ions arriving outside this phase range are either repelled before they enter the entrance hole, or strike the surface of the electrode due to acceleration by the high RF voltage after they have entered the entrance hole.
In the case of the MALDI ion source, ions with different masses are produced from a mixture of sample and matrix, which evaporates and helps ionization of the sample after irradiation by a laser pulse. The ions have different energies as well as different masses, but have the same type of velocity distributions centred on a velocity of several hundred m/s. Consequently, ions having different masses have energies proportional to their masses and ions with the highest mass have the widest energy distribution. For example, ions of mass 10,000 Da, having a maximum velocity of 1200 m/s for their velocity distribution, have energies up to 75 eV, while ions of mass 1000 Da, with the same velocity distribution, have a maximum energy of only 0.75 eV. It becomes increasingly difficult to trap ions having higher masses because the trapping pseudo-potential produced by the RF voltage is inversely proportional to the ion mass, as described in a standard text book on the quadrupole ion trap; for example, xe2x80x9cQuadrupole Storage Mass Spectrometry, R. E. March and R. J. Hughes, John Wiley and Sons, 1989, p.77xe2x80x9d. Thus, a higher RF voltage is required to trap ions of higher mass resulting in narrower acceptance parameters for the RF phase and therefore lower trapping efficiency.
An attempt to overcome these difficulties was made by V. M. Doroshenko et al. and is described in U.S. Pat. No. 5,399,857. The described technique uses an increasing RF voltage, normally which is a linearly increasing RF and starts from zero at the time of ion creation. The RF voltage is initially low enough to allow the ions to enter the trapping region and increases as the ions penetrate deeper into the trapping region. When the ions approach the electrode surface at the other side of the trapping region, the increased RF voltage will already have established a trapping field which is sufficiently strong to trap the ions, and prevent them from being lost by hitting the electrode surface. As described in U.S. Pat. No. 5,399,857, if the ions are generated close to the entrance hole, the initial RF voltage experienced by the ions will be very small because the time required for the ions to enter the trapping region is short compared to the time needed to reach the other side of the trapping region. However, most external ion sources have a relatively long flight path and so the ions require a longer time to enter the trapping region. In this case, the ions experience a relatively high RF voltage at the entrance hole, preventing them from being trapped with high efficiency.
It is an object of the invention to provide a method of trapping ions in an ion trapping device which alleviates the above-mentioned problems.
Accordingly, the invention provides a method of trapping ions in an ion trapping device having a ring electrode and two end-cap electrodes, the method comprising:
(a) forming sample ions in an ion source external to said ion trapping device,
(b) introducing said ions into said ion trapping device through a hole at the centre of a first said end-cap electrode,
(c) retarding said ions by applying to a second said end-cap electrode a retarding voltage relative to said first end-cap electrode and said ring electrode, said retarding voltage being applied before said ions have entered the ion trapping device through said hole,
(d) removing said retarding voltage when said ions are about to change their direction of motion towards said first end-cap electrode, and
(e) establishing an ion trapping field quickly by abruptly applying a radio frequency voltage between said ring electrode and said end-cap electrodes after said ions have been introduced into the ion trapping device.
Before the ions have entered the trapping region of the ion trapping device, the RF voltage is sufficiently small, and preferably zero, that the incident ions do not suffer the afore-mentioned repulsion or acceleration which would result in ion loss and reduce trapping efficiency. Thus, ions are free to enter the trapping region when focussed by the external ion source into the entrance hole at the centre of the first end-cap electrode.
In order to reduce the spread of arrival times of ions having a range of initial energies, it is common to accelerate the ions in the ion source using a high voltage and to decelerate the ions just before they reach the entrance hole. However, although the spread of arrival times can be reduced in this way, the ions may still have a wide range of velocities; for example from 100 m/s to 1,200 m/s after deceleration, and this gives rise to spatial spreading in the trapping region. Therefore, it is preferable to apply an offset voltage to the ion source in order to offset the initial energy of the ions and thereby reduce spatial spreading. For example, application of +24V to the sample shifts the initial energy range between 0.5 eV and 75 eV to the energy range between 24.5 eV to 99 eV and this reduces the velocity range of the ions from 12-fold to only 2-fold and reduces the spatial spread as well.
The retarding voltage applied to the second end-cap electrode is preferably a DC retarding voltage. This forms an inhomogeneous electric field in the trapping volume which reduces the ion energy. The electric field thus produced for ion retardation is roughly quadratic and the ions which have entered the trapping region will be turned back towards the first end-cap electrode at substantially the same times regardless of their energy.
One of the aims of the applied retarding voltage is to increase the time for which the ions remain inside the trapping region and to accept ions with different masses arriving at different times. Another aim is to confine the spatial spread of ions to a region at and around the centre of the trapping region To these ends, the space potential at the centre of the trapping region should be substantially the same as the sample voltage applied to the ion source, so that most of the ions will spend a substantial amount of time at or around the centre of the trapping region. The space potential at the centre of the trapping region is about one fifth of the retarding voltage applied to the second end-cap electrode. Accordingly, the method further comprises applying to said ion source an offset voltage relative to said first end-cap electrode and said ring electrode, said offset voltage having an amplitude of substantially one fifth of said retarding voltage and being applied to said ion source while said ions are being extracted from the ion source. In the above illustration, the sample voltage applied to the ion source is 24V and so, in this;case, the retarding voltage applied to the second end cap would be 120V.
Because the retarding voltage is removed when the ions being repelled are at the point where they have lost most of their energy, i.e. when they are on the point of being turned back towards the first end-cap electrode, the ions will have very low kinetic energies, making it easier to trap those ions using a lower RF voltage.
After the initial energy of the ions has been reduced and the ions are within the trapping volume, the RF voltage is applied quickly to establish the trapping field. On the application of the RF voltage the positions of the ions are very important because the vibrational energy after trapping is proportional to the square of their displacement from the centre of the trapping region. By retarding the ions using the near quadratic electric field produced by the DC retarding voltage, stated above, the vibrational energy of the trapped ions is very effectively reduced. This reduces the requirement for ion cooling, which is usually the next necessary process for the quadrupole ion trap after the trapping process. The trajectories of ions after trapping are relatively stable because the ions are far from the disturbed trapping field around the exterior of the trapping region.
It is preferable to start the RF voltage from the negative part of the voltage cycle. In this case, ions in the trapping region will begin their motion inwardly for an axial component but outwardly for a radial component. By contrast, if the RF voltage were to start from the positive part of the voltage cycle it is likely that ions having relatively high initial energies would be lost by striking the end-cap electrode because the initial direction of the movement is outwardly for the axial component.